The present invention relates to a rechargeable battery pack using a secondary battery.
A circuit as shown in FIG. 6 is known as a rechargeable battery pack 1 using a secondary battery. This secondary battery, indicated by numeral 2, is connected to an external terminal VDD or VSS via a switch circuit 4. Also, an overcharging/overdischarging-and-overcurrent control circuit 3 is connected to the secondary battery 2. This control circuit 3 acts to detect the voltage across, or the current through, the secondary battery 2. That is, the control circuit 3 detects the state of overcharging (i.e., that the voltage across the secondary battery 2 is higher than a given high voltage value) or the state of overdischarging (i.e., that the voltage is lower than a certain voltage value) and the state of an overcurrent (i.e., that the current is greater than a given current value). The control circuit 3 detects overcharging/overdischarging and overcurrent and produces a signal to turn off the switch circuit 4, stopping the supply of energy to the load connected to the VSS or VDD (such as a portable computer using the secondary battery).
Thus, the rechargeable battery pack 1 controls the switch circuit 4 inserted between the secondary battery 2 and the external terminal to prevent overcharging of the secondary battery 2 and an excessive decrease in the storage capacity of the secondary battery 2 due to supply of energy to the load from the secondary battery 2.
Let V1 be the voltage developed across the secondary battery. Let V3 be a voltage used to detect the state of overdischarging. This voltage V3 will be referred to as the overdischarging detection voltage. With the prior art rechargeable battery pack, if the secondary battery voltage V1 decreases below the overdischarging detection voltage V3 as shown in FIG. 7 (A), the overcharging/overdischarging-and-overcurrent control circuit detects the overdischarging and overcurrent, turning off the switch circuit 4. Immediately thereafter, the output voltage V4 between the VSS and VDD decreases to 0 (FIG. 7 (B)). At this time, if the load using this rechargeable battery pack is a computer employing a secondary battery, for example, and if the computer is still operating after the moment the voltage between VSS and VDD decreases to 0, then data loss or a malfunction may occur.